Design Principles and Mechanistic Insights for Ionic Liquid-Catalyzed CO₂ Cyclization Under Mild Conditions

Utilizing carbon dioxide (CO₂) as a renewable C1 building block to produce value-added chemicals represents a key strategy for developing greener and more sustainable chemical processes. Herein, a series of ionic liquids (ILs) with excellent CO₂ activation capabilities was designed and combined with copper(I) chloride to form a synergistic catalytic system. This system efficiently promoted the carboxylative cyclization of propargylic alcohols with CO₂ under mild conditions, such as at room temperature and atmospheric pressure. Among the explored ILs, tetramethyl guanidinium 2,4-dihydroxybenzoate ([TMGH]₃[2,4-DHBA]) was the most effective catalyst, achieving a conversion of 68.9% for the model substrate 2-methyl-3-butyn-2-ol within 12 h. The superior performance of [TMGH]₃[2,4-DHBA] is attributed to the presence of multiple CO₂-philic sites on its anionic scaffold, which significantly enhances the cooperative activation of CO₂. Furthermore, theoretical simulations revealed that the structural features of the anion play a decisive role in CO₂ adsorption and activation, exhibiting a positive correlation with the overall catalytic activity. This work systematically elucidates, from both experimental and theoretical perspectives, the catalytic mechanism of ILs with different cation-anion combinations in CO₂ transformation reactions, providing critical insights for the rational design of highly efficient and mild catalytic systems for CO₂ utilization.